It is well known that microbodies or microglobules of air or gas (defined here as microvesicles), e.g. microbubbles or microballoons, suspended in a liquid are exceptionally efficient ultrasound reflectors for echography. In this disclosure the term of "microbubble" specifically designates hollow spheres or globules, filled with air or a gas, in suspension in a liquid which generally result from the introduction therein of air or gas in divided form, the liquid preferably also containing surfactants or tensides to control the surface properties and the stability of the bubbles. The term of "microcapsule" or "microballoon" designates preferably air or gas-filled bodies with a material boundary or envelope, i.e. a polymer membrane wall. Both microbubbles and microballoons are useful as ultrasonic contrast agents. For instance injecting into the bloodstream of living bodies suspensions of air-filled microbubbles or microballoons (in the range of 0.5 to 10 .mu.m) in a carrier liquid will strongly reinforce ultrasonic echography imaging, thus aiding in the visualization of internal organs. Imaging of vessels and internal organs can strongly help in medical diagnosis, for instance for the detection of cardiovascular and other diseases.
The formation of suspensions of microbubbles in an injectable liquid carrier suitable for echography can be produced by the release of a gas dissolved under pressure in this liquid, or by a chemical reaction generating gaseous products, or by admixing with the liquid soluble or insoluble solids containing air or gas trapped or adsorbed therein.
For instance, in U.S. Pat. No. 4,446,442 (Schering), there are disclosed a series of different techniques for producing suspensions of gas microbubbles in a sterilized injectable liquid carrier using (a) a solution of a tenside (surfactant) in a carrier liquid (aqueous) and (b) a solution of a viscosity enhancer as stabilizer. For generating the bubbles, the techniques disclosed there include forcing at high velocity a mixture of (a), (b) and air through a small aperture; or injecting (a) into (b) shortly before use together with a physiologically acceptable gas; or adding an acid to (a) and a carbonate to (b), both components being mixed together just before use and the acid reacting with the carbonate to generate CO.sub.2 bubbles; or adding an over-pressurized gas to a mixture of (a) and (b) under storage, said gas being released into microbubbles at the time when the mixture is used for injection.
EP-A-131,540 (Schering) discloses the preparation of microbubble suspensions in which a stabilized injectable carrier liquid, e.g. a physiological aqueous solution of salt, or a solution of a sugar like maltose, dextrose, lactose or galactose, is mixed with solid microparticles (in the 0.1 to 1 .mu.m range) of the same sugars containing entrapped air. In order to develop the suspension of bubbles in the liquid carrier, both liquid and solid components are agitated together under sterile conditions for a few seconds and, once made, the suspension must then be used immediately, i.e. it should be injected within 5-10 minutes for echographic measurements; indeed, because they are evanescent, the bubble concentration becomes too low for being practical after that period.
In an attempt to cure the evanescence problem, microballoons, i.e. microvesicles with a material wall, have been developed. As said before, while the microbubbles only have an immaterial or evanescent envelope, i.e. they are only surrounded by a wall of liquid Whose surface tension is being modified by the presence of a surfactant, the microballoons or microcapsules have a tangible envelope made of substantive material, e.g. a polymeric membrane with definite mechanical strength. In other terms, they are microvesicles of material in which the air or gas is more or less tightly encapsulated.
For instance, U.S. Pat. No. 4,276,885 (Tickner et al.) discloses using surface membrane microcapsules containing a gas for enhancing ultrasonic images, the membrane including a multiplicity of non-toxic and non-antigenic organic molecules. In a disclosed embodiment, these microbubbles have a gelatine membrane which resists coalescence and their preferred size is 5-10 .mu.m. The membrane of these microbubbles is said to be sufficiently stable for making echographic measurements.
Air-filled microballoons without gelatin are disclosed in U.S. Pat. No. 4,718,433 (Feinstein). These microvesicles are made by sonication (5 to 30 kHz) of protein solutions like 5% serum albumin and have diameters in the 2-20 .mu.m range, manly 2-4 .mu.m. The microvesicles are stabilized by denaturation of the membrane forming protein after sonication, for instance by using heat or by chemical means, e.g. by reaction with formaldehyde or glutaraldehyde. The concentration of stable microvesicles obtained by this technique is said to be about 8.times.10.sup.6 /ml in the 2-4 .mu.m range, about 10.sup.6 /ml in the 4-5 .mu.m range and less than 5 .times.10.sup.5 in the 5-6 .mu.m range. The stability time of these microvesicles is said to be 48 hrs or longer and they permit convenient left heart imaging after intravenous injection. For instance, the sonicated albumin microbubbles when injected into a peripheral vein are capable of transpulmonary passage. This results in echocardiographic opacification of the left ventricle cavity as well as myocardial tissues.
Recently, still further improved microballoons for injection ultrasonic echography have been reported in EP-A-324.938 (Widder). In this document there are disclosed high concentrations (more than 10.sup.8 /ml) of air-filled protein-bounded microvesicles of less than 10 .mu.m which have life-times of several months or more. Aqueous suspensions of these microballoons are produced by ultrasonic cavitation of solutions of heat denaturable proteins, e.g. human serum albumin, which operation also leads to a degree of foaming of the membrane-forming protein and its subsequent hardening by heat. Other proteins such as hemoglobin and collagen were also said to be convenient in this process. The high storage stability of the suspensions of microballoons disclosed in EP-A-324.938 enables them to be marketed as such, i.e. with the liquid carrier phase, which is a strong commercial asset since preparation before use is no longer necessary.
Similar advantages have been recently discovered in connection with the preparation of aqueous microbubble suspensions, i.e. there has been discovered storage-stable dry pulverulent composition which will generate long-lasting bubble suspensions upon the addition of water. This is being disclosed in Application PCT/EP 91/00620 where liposomes comprising membrane-forming lipids are freeze-dried, and the freeze-dried lipids, after exposure to air or a gas for a period of time, will produce long-lasting bubble suspensions upon simple addition thereto of an aqueous liquid carrier.
Despite the many progresses achieved regarding the stability under storage of aqueous microbubble suspensions, this being either in the precursor or final preparation stage, there still remained until now the problem of vesicle durability when the suspensions are exposed to overpressure, e.g. pressure variations such as that occurring after injection in the blood stream of a patient and consecutive to heart pulses, particularly in the left ventricle. Actually, the present inventors have observed that, for instance in anaesthetised rabbits, the pressure variations are not sufficient to substantially alter the bubble count for a period of time after injection. In contrast, in dogs and human patients, typical microbubbles or microballoons filled with common gases such as air, methane or CO.sub.2 will collapse completely in a matter of seconds after injection due to the blood pressure effect. This observation has been confirmed by others: For instance, S. GOTTLIEB et al. in J. Am. Soc. of Echocardiography 3 (1990) 238 have reported that cross-linked albumin microballoons prepared by the sonication method were losing all echogenic properties after being subjected to an overpressure of 60 Torr. It became hence important to solve the problem and to increase the useful life of suspensions of microbubbles and membrane bounded microballoons under pressure in order to ensure that echographic measurements can be performed in vivo safely and reproducibly.
It should be mentioned at this stage that another category of echogenic image enhancing agents has been proposed which resist overpressures as they consist of plain microspheres with a porous structure, such porosity containing air or a gas. Such microspheres are disclosed for instance in WO-A-91/12823 (DELTA BIOTECHNOLOGY), EP-A-327 490 (SCHERING) and EP-A-458 079 (HOECHST). The drawback with the plain porous microspheres is that the encapsulated gas-filled free space is generally too small for good echogenic response and the spheres lack adequate elasticity. Hence the preference generally remains with the hollow microvesicles and a solution to the collapsing problem was searched.